Everybody knows sand from the summer time on the beach, as construction material or raw material for glass making. Talking about beach sand, did you ever notice that not all the beaches are the same? Some are really wonderful to walk onto while others can be a pain for the feet. And some are white like the snow and others are dark like the night. What about the sand from the desert, is it the same like the one from the beach? What makes them different, why they have so many colors…? Without pretending to be an expert I will try to answer these questions here from the point of view of a sand collector and not of a scientist. So…

…what is sand?

Sand can be defined as naturally occurring granular material having the grain diameter falling in the interval of 0.063 (1/16) to 2.0 mm. In the USA sand is usually classified by grain size into five main categories: very fine (1/16 – 1/8 mm), fine (1/8 – 1/4 mm), medium (1/4 – 1/2 mm), coarse (1/2 – 1 mm) and very coarse (1 – 2 mm).

…where does the sand come from?

Sand can be found on a variety of relief forms from beaches to deserts, lake shores, rivers, but also mountains, quarries, dunes along sea shores, sometimes also the side of the road. But how the sand arrives there depends on the processes that lead to the formation of sand grains. The most important is represented by weathering of the rocks caused by external factors like wind, precipitations, freezing – unfreezing cycles etc.

This method is basically mechanical and the sands resulted bear the physico-chemical characteristics of the mother rock they originate from. Explosive volcanism (pyroclastic process) is the reason for a more ‘dramatic’ formation of sands associated with volcanic activity and formation of igneous rocks of altered chemical composition. The third class is given by rock crushing mechanisms, both caused by rock movements as well as caused by the impact between two rock massifs. Deposition of dissolved minerals from warm seas can be the reason for the formation of sand grains by pelletization using both chemical and biological precipitation processes.

…what is the composition of sand?

The sand composition depends directly on its origins. As an example, sands collected from volcanic islands often have in their composition volcanic tuffs and ashes while the beaches of single atolls are usually made of corals and skeletal remains of tiny sea animals. The mineralogical composition of sands does have also a direct connection to the sands’ color, the mother nature being here extremely generous: white sands with biologic origin, black sands from volcanic islands, reddish hues in arid areas (deserts) etc.

Weathered cross-bedding sandstone layers in Negev desert, Israel

Further classification of sands is based on the consideration whether the sand has resulted from processes related with animal and plants life (biogenic sands) or resulted from rocks transformations (mineral sands). Very often in nature we can find combinations of these two groups – in this case we can speak about mixed sands. Nevertheless, the term carbonate sands is also used when speaking about sands formed by precipitation of calcium carbonate. This term may be very conclusive but it does not make any difference between biological precipitation (calcium carbonate found in skeletons of sea animals) and chemical precipitation (calcium carbonate dissolved in warm sea waters).

Biogenic sands

A biogenic substance is generally defined as a substance resulted from life processes (either animals or plants or both). The term ‘biogenic sand’ refers to sand made of skeletal remains of plants and animals. Most sands of biogenic origin contain skeletal rests of corals, calcium-depositing algae and different small marine animals such as gastropod mollusks (snail-like shells or fragments), bivalve shells, barnacles, foraminifera, sponge spicules, worm tubes etc. The biogenic sands can be distinguished from mineral sands by their high content in calcium carbonate (CaCo3).

Biogenic sand made of corals and gastropod mollusk fragments

Mineral sands

The most common constituent of mineral sands is silicon dioxide present in its quartz form. Silicate sands (based on quartz) are present on beaches, deserts but also rivers, mountains and lakes and, to some extend, on volcanic islands. Beside quartz, another popular group are the sands composed of immature sediments such as granitic rock fragments (which implies that original rock is not very far and the weathering agents did not have sufficient time to alter the original rock). These sands may be called lithic sands and they may contain a huge variety of rock fragments (granite, basalt, sandstone, limestone etc).

Lithic sands from the British coastline (most lithic sands have dark color)

Heavy mineral sands contain those minerals with specific weight higher than 2.9 g/cm³. Due to their stable composition in time, the heavy minerals offer valuable information about the original rocks they are originating from. Eroded from these rocks and transported to the bottom of water bodies, the heavy mineral sands are too heavy to be transported to the shores by the regular waves but they can be found in thin layers on the beaches after some stormy nights. Very common minerals are magnetite (black color, with magnetic properties), garnets (usually pink or red), ilmenite (black) etc.

Another group of sands which is very spectacular and offers impressive macroscopic images is represented by the volcanic sands. This type of sand is generally made of unconsolidated fragments of volcanic debris, usually lava which is cooling off very fast after a volcanic eruption. Volcanic sands can be further classified according to the final characteristics of the lava. When the ejected solid material get into violent contact with gases and steam released by a volcano it is blown apart into pieces of diffeernt sizes. The very small sized material forms the volcanic ash which, by consolidation, lead to the formation of volcanic tuff. Further weathering of volcanic tuff can produce particles of diameters in sand range as depicted in the image below:

Volcanic sand originating from volcanic tuff of Etna volcano, Italy

Mixed sands

The mixed sands contain both mineral and biogenic sand grains and are very common especially on sea shores. However, the group is very difficult to define since the it consists of combinations in different ratios of any of the above described sands (the mineral part can be of igneous, sedimentary or metamorphic origin while the biogenic part can have animal or plant origins).

Mixed sand from South Africa (quartz grains and shell fragments)

Special sands

These are sands of special interest for sand collectors with very often only one major constituent. These sands do not form a scientific group by themselves (they can be either biogenic or mineral) but they are very appreciated for their visual characteristics (color, grain shape etc) so I decided to list them separately, also mentioning the group they belong to.

Olivine sand from Hawaii (mineral sand)

Serpentine sand from Corsica, France (mineral sand)

Foraminifera sand from Bali, Indonesia (biogenic sand)

Blue sodalite sand from Namibia (mineral sand)

Almandine sand – ‘star garnet’ sand from Idaho, USA (mineral sand)

Muscovite sand made of white, common mica (mineral sand)

‘Star sand’ from Japan made of skeleton remains of foraminifera (biogenic sand)

Magnetite sand, almost in pure form (mineral sand)

Hematite sand made of quartz with iron oxide pigment (mineral sand)

After definition, origins and classification, the physical characteristics of sand like grain size, roundness, sphericity, and sorting degree will be described here during the following days so stay tuned and visit again this page if you want to know more about the fascinating sand.

Click on the continent image to see sand micro-photographs from contries belonging to the respective continent. Images will be added on regular basis so check the links regularly to get the latest sand micro-imagery. In addition to that you may also click on the photo camera (top-right side of the screen) to access the Sand Photomicrography library.

EUROPE

ASIA

AFRICA

OCEANIA

NORTH AMERICA

SOUTH AMERICA

The present glossary of terms is meant to provide some general explanations for the more technical, sand-related terms used on the Sand Atlas website. Please consider the descriptions here as being rather general and not really scientific (I am not a geologist nor mineralogist and don’t pretend to be one). The list will be updated regularly so please check this page more often. Some entries may contain photos and links to some example pages for more information. If you’d like to help me add some new terms or correct some possible mistakes, please feel free to contact me at info [at] sand-atlas.com.

A

Almandine

Rock-forming mineral with a deep red color (slightly purple) belonging to the garnet group having the chemical formula: Fe3Al2(SiO4)3. The name almandine (also known as carbuncle) comes from Alabanda, a region in Asia where the minerals were firstly found in ancient times. The term carbuncle is derived from Latin and means “live coal” or burning charcoal. Almandine occurs frequently in metamorphic rocks like mica schist (see also this post for more details).

Arenophile

Arenophile is the term used to describe a person who is interested in collecting sand samples as hobby. The criteria for collecting sand can be various: colors, location, mineralogy (composition) etc. Although in general only sand is collected, sometimes other samples from other materials are collected too such as crushed rocks, mud, soils, etc. Very often people like to swap sand with other collectors and use to gather in online communities such as Sand Forum International.

B

Biogenic

A biogenic substance is generally defined as a substance resulted from life processes. The term ‘biogenic sand’ refers to sand made of skeletal remains of plants and animals. Most sands of biogenic origin contain skeletal rests of corals, algae and different small marine animals such as mollusks (snails, shells), foraminifera, sponge spicules, worm tubes etc. The biogenic sands can be distinguish from mineral sands by their high content in calcium carbonate (CaCo3).

C

Coral

Corals are marine organisms living in compact colonies secreting calcium carbonate to form a hard skeleton. The coral colonies are spread over the oceans mostly surrounding atolls and islands. Skeleton rests can form nice sands much appreciated by sand collectors.

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D

E

F

Fuchsite

Green colored form of muscovite (common mica) with high chromium content (1-5%) and chemical formula: K(Al,Cr)3Si3O10(OH)2.

G

Garnet

Group of minerals used since the old times as gemstone also renowned for displaying the greatest variety of color than any other mineral, occurring in any color (except blue). Most common color is purple red.

Glauconite

Iron silicate from the larger mica group whose name derives from the Greek glaucos (meaning “gleaming” or “silvery” and referring to its blue-green color, which can vary from bluish green to olive green). The chemical formula of glauconite is K0.08R11.33R20.67[(Al0.13Si3.87O10](OH)2.

H

Heavy mineral sands

Sands containing good amounts of minerals such as ilmenite, zirconium, rutile, titanium, etc. and trash minerals such as magnetite and garnet. The heavy mineral sands are to be found especially on the beaches where tidal action brought to the light the heavier minerals.

I

Igneous (rock)

One of the three main types beside sedimentary and metamorphic rocks. The igneous rocks are formed by cooling off and solidification of magma. Further classification divide igneous rocks in intrusive (magma solidifies under crust surface) and extrusive (magma solidifies above crust surface).

J

K

L

M

Maerl sands

Deposits formed by the shells of some specific red algae living mostly off the Brittany and Ireland coasts, primary composed of calcium and magnesium carbonate, but containing also high amounts of magnesium, iron and other elements.

Muscovite

Mineral known also as mica, first use was in caves painting, nowadays it is used in various industrial applications.

Mica

Group of silicate minerals characterized by almost perfect cleavage yielding remarkably-thin laminæ (sheets) which are often highly elastic. Common mica is also known under the name muscovite.

Mud volcanoes

Volcano-like formations having similar characteristics as the “real volcanoes” just that magma is made out of water and clay from the underground layers. This is deposited in the form of mud at the top of the volcanoes, with the eruption of gases coming out like some lazy bubbles.

N

O

P

Phyllite

Phyllite is a foliated metamorphic rock found in the surroundings of stones of the pre-Cambrian epoch. In Tharandt Open-Air Geological Museum we found phyllite in Mohorn-Grund (north of Tharandter Forest). Due to the high content in mica, phyllite has a nice silvery aspect but is very fragile and breaks easily just under the pressure of fingers.

Pitchstone

Pitchstone is a volcanic rock with dull, glassy aspect and very resistant to erosion. In comparison with obsidian (a volcanic rock with similar characteristics), pitchstone contains about 8% water in its structure. The main color of pitchstone is black but, in case of rock formations from Tharandt, Germany, the stone contains small red patches. Their presence in the main block is not yet clearly explained, some theories suggesting that they are foreign rock particles (most probably hematite – a red iron ore mineral) assimilated by the molten lava on its way to the surface.

Porphyry

Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The color of porphyry is usually reddish due to the presence of hematite.

Q

R

Roundness

Measure describing the sharpness of a grain’s corners and edges, regardless of shape. A perfect rounded particle would have the roundness = 1, all the others being included in different roundness classes.

S

Shale

Shale rock (in German: Schiefer) is the name used for a sedimentary rock made of… mud (actually a mix of clay and some other minerals like quartz and calcite). The fine-graded rock can be broken into parallel layers and it is often used for covering the roofs of houses in mountain areas.

Sphericity

Measure of the degree to which the shape of a particle approaches that of a sphere. A perfect spherical grain would have the sphericity factor = 1 but this is rather uncommon in nature as most sand grains have sphericity numbers around 0.7.